Why could the middle Holocene have warmer summers but cooler winters in the Northern Hemisphere?

During the middle Holocene Epoch, the Northern Hemisphere experienced warmer summers and cooler winters primarily due to changes in the Earth's orbital geometry, known as Milankovitch variations.12

These variations altered the seasonal and latitudinal distribution of solar radiation (insolation) reaching the Earth's surface. Specifically, the precession of the Earth's axis meant that 9,000 years ago, the Earth was at perihelion (its closest point to the Sun) near the June solstice, rather than the December solstice as it is today. This configuration increased the intensity of solar energy received during the Northern Hemisphere summer.13

Conversely, this same orbital shift resulted in the Earth being farther from the Sun during the Northern Hemisphere winter, leading to reduced solar radiation and cooler winter temperatures. These competing seasonal changes in insolation created a distinct climatic pattern during the Mid-Holocene Climatic Optimum, where regional temperature trends were driven by the balance of these seasonal extremes rather than a uniform global warming or cooling.13